Test circuit



May 5, 1936. o. cEsAREo v I TEST CIRCUIT Fild Sept. 20, 1954 lNVENTORQCESAREO A TTORNEV Patented May 5, 1936 UNITED STATES PATENT OFFICE TESTCIRCUIT Application September 20, 1934, Serial No. 744,768

11 Claims.

This invention relates to test circuits and particularly to theutilization in such circuits of gaseous conductor or discharge tubes fordistinguishing between inductive and non-inductive circuit conditions.

The gaseous tube employed in the circuits of this invention is a tubefilled with a low pressure gaseous content, such as neon, argon, helium,mercury Vapor or combinations of gases of this group. Such a tube ischaracterized by the fact that the contained gas ionizes and becomesconducting at a predetermined specified potential and once renderedconducting, maintains this condition of conductivity at a lowerpotential and until such time when some other condition is imposed onthe tube, such as opening the output circuit, to stop the ionizationphenomenon.

The present invention has for its object the utilization of such a tubein a test circuit for distinguishing between inductive and non-inductivecircuits.

In its general aspect, this invention utilizes the energy stored in aninductive circuit as the means for imposing on the tube a potential ofsuch a nature as to cause the gas in the tube to become ionized andtherefore conductive and including in the circuit completed by theionization of the gaseous content a signal control device such as arelay.

The present invention finds particular application in telephone systemsof the prepayment type wherein a subscriber is required to deposit acoin or token in the telephone coin box before a desired connection canbe completed. In such systems, the deposit of a coin at a pay stationeffects the completion of a circuit to the central ofiice which includesthe inductive windings of the coin control magnet which constitutes anessential element in the equipment located at the subscribers station.This circuit is completed by the application of ground potential to thetelephone line.

Unscrupulous persons, in an endeavor to defraud the operating companiesand obtain free calls, have resorted to the practice of simulating thedeposit of a coin by artificially grounding the telephone line. Thecircuit to the central office completed in this manner, however, doesnot include the inductive windings of the coin control magnet and istherefore a non-inductive circuit. The present invention utilizes thisfact by providing a test circuit which includes means which is renderedresponsive when associated with an inductive circuit and unresponsivewhen associated with a non-inductive circuit.

In order to disclose its practicability and to more clearly describe itsfunctioning, the present invention is shown in the drawing as applied toa telephone system of the prepayment type, but it is to be understoodthat it is equally applicable to any type of electric circuit, theinductive or non-inductive characteristic of which is to be determined.

lin the drawing Fig. 1 shows a subscribers telephone line of theprepayment type terminating in the usual manner at a central ofiice atwhich there is located an operators cord circuit with which may beassociated the test circuit of this invention. The test circuit of Fig.l utilizes a gaseous tube of the cold cathode type;

Fig. 2 shows the test circuit in which a hot cathode type tube issubstituted for the tube in Fig. 1; and

Fig. 3 is a simplified schematic showing the circuit conditions whichprevail during the test and just prior to the opening of the circuit ontest.

It is believed that the invention will be readily understood from thefollowing detailed description made with reference to the accompanyingdrawing, the figures of which have been generally described above.

Initiating call by coin deposit A subscriber at station A desiring toobtain a telephone connection removes the receiver and deposits a coinID in the coin box located at the station A. The deposit of the coincloses the coin contacts H to ground in the well known manner. A circuitis thereby established extending from grounded battery, winding of linerelay i2, armature and back contact of cut-off relay l3, tip conductor Mof the subscribers line circuit, windings of coin control magnet l5 toground at the contacts ll.

Relay l2 operates in this circuit and causes line lamp IE to be lightedin an obvious circuit. Upon noting the lighted line lamp IS, an operatorat central oifice B inserts the answering plug ll of a cord circuit Cinto the jack I8 associated with the lighted line lamp [6. Cut-off relay!3 now operates in an obvious sleeve circuit and opens the circuit toline relay I2, which relay releases its armature causing line lamp Hi tobe extinguished.

The operator may now test the line for coin ground by actuating key 20.It will be noted that the test circuit generally indicated at T iscommon to a plurality of cord circuits such as C and thatslow-to-release relay 2| is normally energized, the circuit for thisrelay extending to ground on the back contact 22 associated with a key,such as key 20, individual to the last cord circuit located at theoperators position.

Actuation of key 20 associated with cord circuit C opens the energizingcircuit for relay 2| at the contact 23 and at contacts 24 and 25connects the tip oi plug II to conductor 34. At contacts 26 and 21, key20 disconnects the conductors of cord circuit C from the plug At contact28, key 20 connects the positive terminal of battery 29 to conductor 30thereby completing a circuit to the anode of tube 3|.

Relay 2| being slow to release maintains, for a short period of time, acircuit extending from grounded battery 32, resistance 33, armature andfront contact of relay 2|, conductor 34, contacts 24 of key 2|], tipsprings of plug I1 and jack l8, conductor l4, windings of coin controlmagnet l5 to ground at coin contacts II. This circuit is maintained fora period of time sufiiciently long to fully build up the current throughthe inductance of the coin control magnet coils.

It will be understood that the potentials of batteries 32, 35, and 29are fixed so as to be just below the minimum critical operating point ofthe tube so that the tube does not flash under normal conditions of notest.

When relay 2| releases, the inductive circuit previously traced throughthe coin control magnet IE is opened whereupon the energy stored in themagnet coils is released causing a potential reversal and rise oncathode 31, this cathode being connected to the inductive circuit by wayof conductor 39. Thisrise in potential on cathode 31, due to thepotential rise generated by the breaking of the circuit including theinductance of the coin control magnet, raises the potentials on the tubeabove the critical breakdown point, causing the gas in the tube toionize and become conductive so that relay 40 operates in a circuitextending from the battery 29, contacts 28 of key 20, conductor 30,winding of relay 40, anode of tube 3| to the tube cathode 38. Relay 40operates in this circuit completing an obvious circuit for signal lamp4|.

The lighting of lamp 4| indicates to the operator that the test circuithas been associated with an inductive circuit, and in the caseillustrated, that a coin has been deposited in the coin box. The key 20may then be restored and the call extended in accordance with usualpractice. Restoration of key 20 opens contact 28 thereby interruptingthe anode circuit of the tube, ionization stops and the tube is renderedinert.

Line fraudulently grounded Should the subscriber at station A attempt toobtain service without depositing a coin in the coin box by simulating,this act by grounding the line, the test is made in the same manner ashereinbefore described.

It will be noted that fraudulently grounding the line at D, for example,causes the line relay |2 to operate in an obvious circuit which in turncauses line lamp Hi to be lighted. The operator proceeds to answer thecall in the usual manner and tests the calling line by the actuation ofa key 20 associated with the cord she uses in answering the call.

Key 20 performs the same functions as hereinbefore described inconnection with a call initiated in accordance with prescribed practice.It will be noted, however, that the inductive 'windings of the coincontrol magnet 5 are not included in the tip circuit being open toground due to the absence of a coin in the coin box; consequently, thecircuit extending from battery 32, resistance 33,.armature and frontcontact of relay 2 I conductor 34, contacts 24 of key 20, tip springs ofplug I1 and jack I8, conductor M to ground at D is a non-inductivecircuit, so that when relay 2| releases to open this circuit, there isno change of potential on the control cathode 31 and consequently thereis no ionization of gas in the tube 3|, which accordingly remains inert.The circuit to relay 4!] is not completed and lamp 4| remains unlighted.

Upon making the test and noting the failure of lamp signal 4| to light,the operator immediately recognizes that an unstandard condition existson the line, that is, that the line has been fraudulently oraccidentally grounded. When such a condition exists, the operator doesnot complete the connection.

With reference to Fig, 2 of the drawing, tube |3| corresponds to tube 3|of Fig. 1, relay 2| to relay 2|, relay I40 to relay 40 and lamp signal|4| to 4|. It is believed unnecessary to repeat the description madewith reference to Fig. 1 in describing the operation of the test circuitshown in Fig. 2. The operation of the systems is identical. The testcircuit of Fig. 2 distinguishes from that of Fig. 1 in utilizing a hotcathode type of tube in place of the cold cathode tube of Fig. 1.

The cathode of tube |2| is continuously heated and the grid serves asthe control element. When this test circuit is associated with aninductive circuit and the test is made, the breaking of the inductivecircuit causes a. reversal and rise of potential on the grid element ofthe tube |3|, causing the tube to flash and become conductive. Relay I40thereupon operates in the plate or anode circuit of the tube, causingthe signal lamp |4| to be lighted. When the circuit tested isnoninductive, there is no reversal of potential on the grid, the tuberemains inert and no signal indication is evidenced.

From the foregoing it is apparent that the present invention provides asimple, practical and accurate arrangement for discriminating between aninductive and a non-inductive circuit to ground which utilizes eitherhot or cold cathode gas-filled tubes, the tubes being arranged tofunction on the potential rise created when an inductive circuit isbroken.

. What is claimed is:

1. In combination, an electric circuit capable of having either aninductive or a non-inductive characteristic, a test circuit includingmeans for discriminating between inductive and non-inductive circuits,said means comprising a gas-filled tube, and means for associating saidelectric circuit with said test circuit to be tested thereby. 2. Incombination, an electric circuit capable of having either an inductiveor a. non-inductive characteristic, a test circuit including means fordiscriminating between inductive and non-inductive circuits, said meanscomprising a gasfilled tube having a control electrode normallymaintained at a certain predetermined potential, and means forconnecting the control electrode of said tube with said electric circuitin such a manner that the potential of said electrode is altered andsaid tube flashes when said electric circuit has an inductivecharacteristic.

3. In combination, an electric circuit including an energy storingdevice, a test circuit including a gas-filled tube having a controlelectrode normally maintained at a certain predetermined potential, asource of energy, means for connecting said energy storing device withsaid source of energy and with said control electrode, means forsubsequently disconnecting said energy storing device from said sourceof energy whereupon the energy stored in said storing device due to itsconnection with said source of energy is released to alter the potentialof said electrode and cause said tube to flash.

4. In combination, an electric circuit including an inductive winding, atest circuit including a gas-filled tube having a control electrodenormally maintained at a certain predetermined potential, a source ofenergy, means for connecting said in- .ductive winding with said sourceof energy to permit a build-up of current in said winding, said meansalso connecting said winding with said control electrode, and means forsubsequently breaking the connection between said source of energy andsaid winding whereupon the decay of current in said winding causes areversal of potential on said control electrode and the tube flashes.

5. In combination, an electric circuit including a coil, a test circuitincluding a gas-filled tube having a control electrode normallymaintained at a certain predetermined potential and an anode, a signalcontrol device connected to said anode, a source of energy, means forconnecting said coil with said source of energy to permit a build-up ofcurrent in said coil, said means also connecting said coil with saidcontrol electrode and means for subsequently breaking the connectionbetween said source of energy and said coil whereupon the decay ofcurrent in said coil causes a reversal of potential on said electrodeand said tube flashes causing said signal control device to operate inthe anode circuit of said tube.

6. In combination, a circuit having an inductive characteristic, acircuit having a non-inductive characteristic, a test circuit includingmeans for distinguishing between the characteristics of said circuits,said means comprising a gas-filled tube which functions to actuate asignal when said first circuit is tested and which remains inert whensaid second circuit is tested, and means for associating said testcircuit with said other circuits.

7. In combination, a line circuit including an inductive winding, asource of energy, a test circuit including a gas-filled tube having acontrol electrode and having a critical operating point, the potentialson said tube being adjusted to a value. below the critical operatingpoint, means for associating the control electrode of said tube withsaid line circuit and connecting said energy source to said inductivewinding, a signal control relay, and means for subsequently breaking thecircuit from said energy source to said inductive winding whereupon theenergy stored in said inductive winding releases and raises thepotential of said control electrode beyond the critical operating pointof said tube, whereupon said tube flashes and completes a circuit tosaid signal control relay.

8. In combination, a gas-filled tube having two cathodes and an anode.all of said electrodes being electrically biased in such a manner as torender said tube normally inert, an energy storage device connected inparallel with one of said cathodes to the biasing source for saidcathode, means for disconnecting the biasing source from said lastmentioned cathode and said energy storing device for causing the energystored in said device to be released and to effect a reversal ofpotential on said last mentioned cathode whereupon said tube is renderedconductive, and a signal control relay in the anode circuit of saidtube.

9. In combination, a telephone line of the prepayment type having a coinreceptacle associated therewith, said line being capable of beinggrounded through the coin control magnet by the deposit of a coin, ordirectly by fraud or accident, a test circuit including means fordiscriminating between the coin ground and the direct ground, said meanscomprising a gas-filled tube which functions when the line tested iscoin grounded and which remains inert when the line. test is directgrounded, and a signal control device controlled by said tube.

10. In combination, a telephone line having associated therewith a coinreceptacle including a coin control magnet, a test circuit including agas-filled tube having a control electrode normally maintained at apredetermined potential, said test circuit being located at. a centralofiice, a source of energy, means at the central oflice for connectingthe control electrode of said tube with the coin control magnet of saidtelephone line after said line has been seized by the deposit of a coinin the coin receptacle and for connecting said magnet with said sourceof energy, and means for subsequently breaking the connection betweensaid magnet and said source of energy whereupon and as a consequencethereof, the potential on said control electrode is altered and saidtube flashes.

11. In combination, a telephone line terminating at a central ofiice,means for grounding said line through an inductive winding, means atsaid central oflice for completing a connection to said line, a testcircuit at said central ofiice, said test circuit including a gas-filledtube having a control electrode, a source of energy normally associatedwith said control electrode to maintain said electrode at a certainpredetermined potential, means for connecting the inductive winding ofsaid line with said source of energy and with said control electrode,and means automatically operating a predetermined period of time afterthe connection of said inductive winding with said source of energy andsaid control electrode for breaking the connection between said sourceof energy and said inductive winding, whereupon the potential on saidcontrol electrode is altered and said tube is rendered conductive.

ORFEO CESAREO.

